Project Summary/Abstract Cholera is an acute dehydrating diarrheal disease caused by infection with Vibrio cholerae. It is endemic in over 50 countries, affecting up to 3 million people and causing more than 100,000 deaths annually worldwide. Currently available oral cholera vaccines (OCV) achieve a lower efficacy and duration of protection in young children compared to that seen in older children and adults, possibly due to the inability of young children to mount polysaccharide-specific antibody responses. We have recently reported that mucosal-associated invariant T (MAIT) cells are activated in cholera and are associated with higher class-switched V. cholerae polysaccharide-specific antibody responses. In pilot studies, we have identified a subset of MAIT cells that express genes associated with B cell help. Additionally, in preliminary in vitro experiments, we show that MAIT cells can induce B cells to differentiate and produce antibodies. Thus, we hypothesize that a subset of MAIT cells, when activated following infection or vaccination, undergo clonal expansion and provide help to B cells through MR1-dependent and -independent interactions to enhance polysaccharide-specific antibody production. In collaboration with the International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDR,B), we propose to determine the role that MAIT cells play in the adaptive response against V. cholerae infection and vaccination. In Aim 1, we will characterize the clonal expansions of MAIT cells during human V. cholerae infection and oral cholera vaccination. We will test the hypothesis that there is a subset of MAIT cells that express factors consistent with B cell help, and that this subset has lower activation and expansion in young child vaccinees compared to older child vaccinees and infected young children. In Aim 2, we will determine the mechanisms through which MAIT cells affect B cell differentiation and antibody production. We will test the hypotheses that MAIT cells provide help to B cells through both MR1-dependent and MR1- independent (cytokine-mediated) interactions with B cells, and that MAIT cells of young children have deficits in one or more of these mechanisms compared to MAIT cells in older children. At the completion of these studies, we will have gained new information on the capacity of MAIT cells to impact polysaccharide-specific antibody responses, which are associated with protection against cholera. This information has the potential to critically inform the development of better vaccine strategies targeted at preventing cholera and other enteric infections in young children.